In published International Patent Application WO 89/09501 (PCT/GB89/00330) there is shown a planar antenna comprising two or more moulded planar members. A first planar member is shaped to form an array of horns, each of which is coupled into a waveguide system in a second planar member. In FIG. 7 of WO/89/09501, for example, the waveguide system comprises a network of open-topped channels 111 in planar member 11. The member 11 is formed by resin moulding and metallising. In order to convert the open-topped channels 111 into closed waveguides, a metal shim 12 is sandwiched between the planar member 10 and 11, the shim 12 being slotted at 120 to form coupling slots between the horns and the waveguide system.
This type of construction gives excellent antenna properties, but is not optimised for high volume, low cost production. There is a considerable amount of assembly work, and there can be problems in achieving dimensional accuracy and a good mechanical bond in sandwiching the parts together.
An object of the present invention is to provide an antenna which overcomes or mitigates these problems.
After moulding the planar members it is necessary to (a) form a metallised coating on the surfaces of at least the horn cavities and the waveguide channels and (b) secure the plate-like elements together face-to-face; these steps can potentially be carried out in any order. These operations must meet a number of requirements: the horn cavities and waveguide channels must be located relative to each other to a high degree of accuracy, the metallisation must be free from gaps and breaks to prevent loss of microwave energy by leakage, and the assembly must be mechanically strong and free from the risk of long-term deterioration caused for example by reaction between incompatible materials. At the same time, it is desired to achieve low cost, high volume production.